The present invention relates to an optical information recording medium for recording and/or reading information optically; and, more particularly, the invention relates to a multi-layered optical disk having an improved recoding/reading accuracy.
One way to achieve an optical disc having a high capacity is by use of a multilayered medium having plural laminated information recording layers. As a read-only disk, a dual-layered DVD-ROM has been proposed. As for a rewritable dual-layered medium, the Japanese Journal of Applied Physics, Vol. 38, pp. 1679 to 1686 (1999), introduces such a technique therein, for example. In these techniques, recording layers are built up with an interval of several 10 xcexcm, and recording/reading of information is effected by focusing optical spots onto each layer. Information on a side opposite that which receives the incident light is recorded/read as the light passes through the layer on the side of the incident light. When reading information from an inner layer, the light passes through the layers on the side of the incident light twice.
These techniques can increase the recording capacity of a medium of the same size to about twice.
JP-A-21720-1993 discloses a triple-layered recordable medium using a high transmittance organic recording film. In accordance with this method, the transmittances of the three layers are 70%, 80% and 90%, respectively, and the transmittance of a recording mark thereof is 100%. By detecting the amount of transmitting light, data recorded on the three layers are read at the same time. The method is capable to increasing the recording density and data transfer rate by about three times.
As for an optical disk using a nonlinear optical layer, a method of photon super-resolution has been proposed. Several methods using this technique have been proposed, and the Japanese Journal of Applied Physics, Vol. 32, p. 5210, discloses one of them, for example. The described method is characterized in that, by providing a mask to block a part of the optical spot so as to transmit light only through an unmasked portion thereof, the effective spot diameter is reduced, thus increasing the density and capacity of the optical disk. Specifically, in the process of photon super-resolution, when the light focuses on a film, the light transmittance of the light of higher intensity is increased, whereas the reflectance of the non-focused portion is high. In known techniques based on the use of photon super-resolution, a medium has reflective films, and the transmittance of the medium itself is always substantially 0%.
The abovementioned techniques, however, have some inherent problems. In recordable and rewritable optical disks, in particular, it is difficult to form a dual-layered medium that secures a satisfactory process margin in consideration of mass productivity and the products or margins for recording/reading conditions. This is because it is difficult to achieve an optimum optical design that allows for high signal modulation in both layers. To increase the signal quality obtained from the light-incident side, the transmittance of the layer should be lowered, and it is better to increase the reflectance and create larger differences in reflectance between a marked portion and a spaced portion. On the other hand, however, for the layer of the farthest side, the higher the transmittance of the layer on the light incident side is, the higher will be the signals that can be received. As such, as for setting the transmittance of the light incident side layer, the signal needs to be shared by two layers, because the optimum transmittance is in conflict for both layers. More details thereof will be described hereinbelow.
Hereinbelow, a dual-layered phase change medium will be described in which L0 denotes a layer on the side facing the incident light, L1 denotes a layer on the other side side thereof, Rc denotes the disk reflectance of a crystal state, and Ra denotes the amorphous state thereof. Rc and Ra of layers L0 and L1 are indicated respectively as Rc0, Ra0, Rc1, and Ra1. The amount of reflected light/incident light, i.e., the drive reflectance, is indicated as Rcd and Rad, and the transmittance of layer L0 is indicated as T0.
Suppose T0=60% and (Rcd, Rad)=(15%, 2%). The reflectance takes on values close to the reflectance of a phase change disk that is currently produced. It is desirable to obtain the same amount of signal from layers L0 and L1. Calculating reflectance, while taking the above into consideration, a setting value for the reflectance of the layer L1 is (Rc1,Ra1)=(41.7%, 5.6%). However, it is difficult to design a disk for a phase change medium that is capable of overwriting that has a reflectance of 40% or higher. If T0 is set higher than 60%, the reflectance and light absorption of the layer L0 is lowered significantly, and it becomes impossible to obtain a desirable property at the layer L0. Moreover, it is necessary that the transmittance is substantially the same in the crystalline state and the amorphous state for the following reason: when the layer L0 has a marked portion and an unmarked portion, and if the light spot passes on a border of two areas of the layer L0 as the light reads the layer L1, the direct current element and amplitude of the signal in reading the layer L1 fluctuate, thereby causing an increase in jitter or the error rate. Therefore, any accidental error of the transmittance for those two states should be suppressed to less than 5 to 10%. However, maintaining a translucent transmittance with the range is difficult when considering the processing margin.
Moreover, while a dual-layered medium generates problems, such as the above, it is almost impossible to achieve a recordable/rewritable optical disk having three or more layers. The triple-layered recordable disk technique described above detects transmittance. In this method, however, optical systems need to be located above and below the disk. Such a structure makes it difficult to adjust the optical systems, thereby lowering the production margin of the drive. Moreover, the method is not applicable to a rewritable disk.
In the super-resolution technique, the effective spot diameter can be made smaller, thus allowing for a higher density. However, the technique has drawbacks, as follows: A. When considering the processing margin for mass productivity, it is difficult to make the size of the light transmitting portion constant over the entire surface of the disk. B. In an optical disk, the signal to noise ratio S/N becomes an issue; and, in this regard, the area of an effective spot as a part of the spot diameter determines the signal level, while the spot diameter irradiating the disk determines the noise, whereby the signal is increased for a short mark, but the overall S/N, including the one for a long mark, is lowered.
In view of the above, the transmittance of the layer L0 should be high at least while reading the layer L1 in consideration of the layer L1. When reading the layer L1, a signal for reading the layer L1 is determined by a square of the transmittance of layer L0, that is, T02. The value of the obtained signal should be no lower than a half of the signal obtained from a single layer L1, thus a desirable value is expressed as:
xe2x80x83T02xe2x89xa750%
∴T0xe2x89xa771%xe2x80x83xe2x80x83EXPRESSION 1
In the case of a triple-layered medium, the signals are determined by a square of the transmittance of layer L0 when reading the layer L1, i.e., T02, and a product of the squared transmittance of layers L0 and L1, i.e., T02T12, when reading the layer L2. In this case, each signal for reading layers L0 and L1 is desirably expressed by:
T02xe2x89xa7⅔=67%
∴T0={square root over ({fraction (2/3)})}≈82%xe2x80x83xe2x80x83EXPRESSION 2
T02T12xe2x89xa7⅓=33%
∴T1={square root over ({fraction (1/2)})}=71%xe2x80x83xe2x80x83EXPRESSION 3
When the expressions are generalized, the transmittance for reading a Jth layer of an n-layered recording medium can be expressed as:                                           ∏                          i              =              l                                      j              -              l                                ⁢                      T            i            2                          ≥                              n            -            j            +            l                    n                                    EXPRESSION        ⁢                  xe2x80x83                ⁢        4            
where the i-layer and the j-layer used herein mean a laminated film interposed between a substrate and a spacer layer, or a laminated film interposed between a spacer layer and another spacer layer, the i-layer or the j-layer being composed of a lower protective layer, a recording layer, an upper protective layer, a nonlinear optical layer or a reflective layer.
However, as described above, it still has a high transmittance, thereby making it difficult to design the layer L0. Such a problem can be solved by producing a medium where the transmittance is lowered and the reflectance is enhanced as the light focuses thereon. Such a mechanism will be described later.
In the above-described Expression 4, the transmittance of the first layer to the jxe2x88x921th layer is dealt with all together, but it is desirable to design the structure in such a manner that a signal is equally divided among respective layers. In the case of a triple-layered medium, for example, T0 and T1 fulfilling Expressions 2 and 3 are defined as:
T0xe2x89xa7{square root over ({fraction (2/3)})}
T1xe2x89xa7{square root over ({fraction (1/2)})}xe2x80x83xe2x80x83EXPRESSION 5
By generalizing the expression, the transmittance Ti of the ith layer only needs to satisfy the following Expression 6:                              T          i                ≥                                            n              -              i                                      n              -              i              +              l                                                          EXPRESSION        ⁢                  xe2x80x83                ⁢        6            
Moreover, it is possible to design a medium that will secure a processing margin if the transmittance is 50% or less when the light is focusing. In this case, signals on layers further than a light-focusing layer as seen from the light incident layers are not read, and thus, it is not necessary to consider differences in transmittance of crystal and amorphous states in such a case, as described above. The transmittance is low enough to make the design of a medium easier, while securing the processing margins.
In the present specification, the phrase xe2x80x9cwhen the light is focusingxe2x80x9d refers to a case when a light spot diameter on a film surface becomes 105% or less of the size of a minimum beam constriction of the optical system of concern. The term xe2x80x9cspot diameterxe2x80x9d used herein means a diameter providing an intensity of 1/e2 of the central intensity when the spot of the light is approximated to the gaussian distribution. When the spot diameter spreads by 5%, the central intensity is about 90%, whereby, it is considered within a margin of the mechanism described below.
A medium that changes transmittance and reflectance, as described above, can be achieved by using a substance whose optical property changes depending on the energy density of the light applied to the layer L0, i.e., by using a nonlinear optical layer. When the nonlinear optical layer is provided between the L0 recording film and L1 recording film, the nonlinear optical layer should be composed of a material that is transparent or translucent when the light is not focused on the L0 recording film, and has a higher reflectance when the optical spot focuses on the L0 recording medium, compared to a case where the light is not focused. Such a change occurs due to absorption of the light. That can be achieved by either using a photon mode or the heat generated by the light absorption. The change should occur depending on the light power density applied to the substance. In order to read the layer L1 immediately after reading the layer L0, the change has to return to an original state within a certain period of time, and the transmittance of the layer L0 has to be high again. It is desirable that it returns to normal naturally during one disk revolution, for example. If the change occurs due to heat, the temperature should return to the original during one disk revolution so as to reverse the change to the original state.
The mechanism is not only applicable to a dual-layered medium, but is also applicable to a multilayered medium having two or more layers.
FIG. 1 illustrates the mechanism. When light of high power density does not irradiate the nonlinear optical layer 104, i.e., when there is no incident light, and when recording and reading the layer L1, the reflectance of the nonlinear optical layer 104 is low, while the transmittance thereof is high. On the other hand, when recording/reading the layer L0, a portion 110 thereof that is irradiated with light becomes metallic, thereby increasing the reflectance thereof.
The above-described object is achieved by providing a nonlinear optical layer between a first recording film (L0 layer) and a second recording film (L1 layer). The nonlinear optical layer has a property such that the transmittance thereof is higher than the reflectance when the light is not focusing, and the reflectance is higher than the transmittance when the light is focusing. The recording layers are not limited to two layers, and the structure is applicable to a multilayered recording medium having more than two recording layers.
The following materials may be used as a nonlinear optical layer: a) thermochromic material, b) transition metal oxide exhibiting a semiconductor-metal transition, c) garnet, and d) magnetic semiconductor.
The thermochromic material changes wavelength dependency of reflectance and transmittance reversibly according to the temperature. One example thereof is a material of triphernylmethane dye. A super-resolution optical disk using the aforementioned material is disclosed in the Japanese Journal of Applied Physics, Vol. 39, pp 752 to 755 (2000).
A semiconductor-metal transition is known to occur with temperature, pressure, and a compound composition ratio as its variables. In this case, a material causing the transition based on the temperature thereof is selected. Such a material may include an oxide of Ti, V, Cr, Mn, Fe, Co, Ni and Cu. The heat dependency of electronic properties of these materials is described in, for example, Solid State Physics, Vol. 21, pp 1 to 113 (1968). If the material is solely used, an optical property, such as the refractive index thereof before and after the transition, does not change much in the wavelength range of visible light used in the current optical disks. A fact regarding VO2 is reported in Physical Review, Vol. 172, pp. 788 to 798 (1968). In order to solve the problem, free electrons generated in the transition metal oxide due to the transition are injected into another material A so as to change the optical response of the material A. In this case, the material that changes its optical response by the injection of an electric charge may be a metal or a semiconductor. In particular, when a semiconductor is used, an electric charge is injected into a conduction band so as to increase the number of carriers compared to that before the transition, thereby increasing the effect thereof. In order to inject the electric charges efficiently, the Fermi energy level of the material A should be smaller than the Fermi energy level of the transition metal oxide indicating the transfer. Moreover, the injection of the electric charge is conducted through an interface. Thus, the larger the area of the interface is, the easier it will be to inject the electric charge. Therefore, more electric charges can be injected if the material A and the transition metal oxide are formed in a multilayered film structure.
Next, a case of using a magnetic material will be described. Among the magnetic materials, there is a kind of material that exhibits a magnetic transition due to heat while changing the optical property simultaneously. Garnet, in particular, exhibits a strong tendency for this change. FIG. 2 shows a temperature dependency of the transmittance of a bulk crystal of garnet having Ga doped thereinto. The wavelength of the light used herein is 400 nm. In FIG. 2, the transmittance decreases drastically at around 120xc2x0 C. The Curie temperature of the material is about 120xc2x0 C., and thus, a change in transmittance occurs due to the magnetic phase transition.
When a magnetic semiconductor is used, a band structure change due to the magnetic property contributes a great deal to a change of the optical property. The temperature dependency of the optical property of the magnetic semiconductor is described, for example, in Semiconductors and Semimetals, Vol. 25, pp. 35 to 72 (1988). The magnetic semiconductor of this kind includes a material described as RMnM, where R is a simple substance or mixture of Cd, Zn, Hg, and Pb, and M is O, S, Se, and Te. RMnM may be used as a simple substance, or it may be mixed with other materials in some cases.
When the above-described nonlinear optical material is applied to a multilayered disk, it is designed in such a manner that the transmittance is high when no focused light is applied, whereas it becomes low when the focused light is applied. In particular, when such material is applied to a phase change disk, a phase change recording film absorbs the light, and thus, it is desirable to design the material to have an absorption factor of substantially 0 when the transmittance thereof is high. In this case, however, it is impossible for the nonlinear optical material to indicate a transmittance change due to the light absorption. This problem can be solved by transferring heat from a film near the nonlinear optical layer that absorbs light, if the nonlinear optical material indicates a transmittance change by heat. The film for absorbing the light may be a recording film, like a phase change film, as described above, or it may be formed by laminating a film of metal or semiconductor within the disk. In order to transfer heat efficiently, the distance between the light absorbing film and the nonlinear optical material has to be shorter, and the thickness of the metal film or semiconductor therebetween needs to be from 0 nm to 50 nm.